1. FIELD OF THE INVENTION:
The present invention relates to a liquid crystal display device which is used as a display of a computer, a TV set, and the like, for example, and includes switching elements such as thin film transistors (hereinbelow, referred to as TFTs) as addressing elements.
2. DESCRIPTION OF THE RELATED ART:
FIG. 11 is an exemplary equivalent circuit of a conventional TFT type liquid crystal display device using TFTs as switching elements. Pixel electrodes 6 are formed in a matrix, and TFTs 1 as switching elements are connected to the respective pixel electrodes 6. A gate electrode of each TFT 1 is connected to a gate line 2 as a scanning line so that the TFT 1 can be driven under control of a gate signal input to the gate electrode via the gate line 2. A source electrode of the TFT 1 is connected to a source line 3 as a signal line so that a data (display) signal can be input to the corresponding pixel electrode 6 via the TFT 1 when the TFT 1 is being driven. Such gate lines 2 and source lines 3 are provided perpendicular to each other along the peripheries of the pixel electrodes 6 arranged in a matrix. A drain electrode of each TFT 1 is connected to the corresponding pixel electrode 6 and a supplemental capacitance (Cs). A counter electrode of the supplemental capacitance is connected to a common line 4 (hereinbelow, referred to as a Cs line). Liquid crystal (C1c) interposed between the pixel electrode 6 and a counter electrode 18 is thus driven.
In order to reduce power consumption of such an active matrix liquid crystal display device, a technique of increasing the aperture ratio of a liquid crystal display device by overlapping pixel electrodes with lines via an interlayer insulating film is disclosed in Japanese Laid-Open Patent Publication No. 6-160900. According to this technique, however, the orientation of the liquid crystal is disordered due to a failure in rubbing at steps formed by lines and TFTs, and crosstalk arises due to a parasitic capacitance generated between the interlayer insulating film and the pixel electrodes. Such troubles badly influences the display. The above publication neither discloses nor suggests any means for solving these problems.
FIG. 12 is a plan view of one pixel portion of an active matrix substrate of a liquid crystal display device 600 proposed by the applicant of the present application in Japanese Patent Application No. 7-206367for solving the above problems.
Referring to FIG. 12, the gate lines 2 and the source lines 3 as shown in FIG. 11 are formed on a transparent substrate to run perpendicular to each other. The TFT 1 is formed as a switching element in the vicinity of each of the crossings of the gate lines 2 and the source lines 3, and is connected to the corresponding pixel electrode 6 through a connection line 5 via a contact hole 7 formed through an interlayer insulating film (not shown). The connection line 5 overlaps a supplemental capacitance (Cs) line 4 via a gate insulating film (not shown), forming a supplemental capacitance. The pixel electrode 6 overlaps the adjacent gate lines 2 and source lines 3 via the interlayer insulating film. With this configuration, the aperture ratio of the resultant liquid crystal display device improves, and disclination can be suppressed since the electric field generated by the lines 2 and 3 is shielded. Moreover, when the gate and source lines 2 and 3 are formed of a conductive light-shielding material such as metal, they can be used as light-shielding films between the pixel electrodes 6. The parasitic capacitance can be reduced by increasing the thickness of the interlayer insulating film to about 2 xcexcm. Conventionally, the vertically adjacent pixel electrodes 6 in the source line direction are generally separated from each other along and above the corresponding gate line 2, as shown in FIG. 12 and as disclosed in the Japanese Laid-Open Patent Publication No. 6-160900.
The active matrix substrate with the above configuration is attached to a counter substrate with a liquid crystal layer interposed therebetween, so as to form a liquid crystal display device. In order to realize a color display in such a liquid crystal display device, color filters are generally formed on the counter substrate. A black matrix is generally provided on the color filters formed on the counter substrate to prevent color mixing and light leakage. In order to reduce production cost, however, omitting such a black matrix has been proposed in the above-mentioned Japanese Patent Application No. 7-206367, for example.
When the black matrix is omitted, the gate and source lines 2 and 3 serve as the light-shielding films for spaces between the pixel electrodes 6. However, a peripheral region surrounding a display region (the region where the plurality of pixel electrodes 6 are formed) of the liquid crystal panel is not shielded. In the peripheral region, shielding of light from a backlight is required especially in the case of a black display. Otherwise, leakage of light from the backlight from the peripheral region will lower the display quality. Moreover, this light shielding should preferably be in such a level as to obtain a light transmittance of 0.1% or less, in consideration of the contrast ratio of the resultant liquid crystal display device.
Omitting the formation of a light-shielding film on the counter substrate is an effective way of reducing production-cost of the liquid crystal display device as described above. Therefore, the development of a liquid crystal display device where light leakage from the peripheral region surrounding the display region can be prevented without forming a light-shielding film on the counter substrate has been sought.
As a solution to the above problem, the applicant of the present application has formed a light-shielding pattern in the peripheral region as shown in FIG. 13. More specifically, a liquid crystal display device 700 has a light-shielding pattern 30 with a line width of 1 mm to several millimeters formed in the peripheral region located between the display region and external driving circuits 22 and 23 of an active matrix substrate. With this configuration, light leakage from the peripheral region can be sufficiently reduced.
FIG. 14 is a layout showing a boundary between the display region and the peripheral region. Specifically, an outermost one of the pixel electrodes 6 located lowermost of the display region shown in FIG. 13 and the portion of the light-shielding pattern 30 adjacent to the outermost pixel electrode 6 are shown. The light-shielding pattern 30 formed in the peripheral region overlaps the source lines 3 extending to the peripheral region. This increases the parasitic capacitance in the peripheral region. Moreover, when the light-shielding pattern 30 is formed so as to short-circuit with the outermost gate line 2 corresponding to the outermost pixel electrode 6, the width of the outermost gate line 2 becomes 1 mm to several millimeters, while that of the other normal gate lines is only several micrometers to several tens of micrometers. This further increases the parasitic capacitance, and the time constant becomes so large due to the increased parasitic capacitance that the outermost gate line 2 cannot be driven sufficiently. In the case of a liquid crystal display device with a diagonal of 12 inches, for example, the parasitic capacitance may become nearly ten times as large as the supplemental capacitance of the normal gate lines 2. This causes problems in both the output capacity of the driver and the time constant of the lines.
In order to avoid the above problems, the light-shielding pattern 30 needs to be separated from the outermost gate line 2. For this separation, a gap 31 should be formed therebetween as shown in FIG. 14. This causes another problem of light leakage from this gap. Such light leakage is visually recognizable enough to lower the display quality even if the gap is a minute slit with a width of several micrometers to about 10 xcexcm. It is therefore important to layout the light-shielding pattern 30 so as not to form such a gap 31 in order to enhance the display quality.
An objective of the present invention is to provide a liquid crystal display device where production cost can be reduced without lowering the display quality.
The liquid crystal display device of this invention including an active matrix substrate, a counter substrate having a counter electrode, and a liquid crystal layer interposed between the active matrix substrate and the counter substrate, further includes: a plurality of scanning lines; a plurality of signal lines formed perpendicular to the plurality of scanning lines; switching elements formed in the vicinity of crossings of the plurality of scanning lines and the plurality of signal lines; a plurality of pixel electrodes connected to the corresponding signal lines via the switching elements; and a plurality of supplemental capacitance lines formed in parallel with the corresponding scanning lines, wherein the pixel electrodes are formed over the corresponding scanning lines, and divisions of the adjacent pixel electrodes along the signal lines are located above the corresponding supplemental capacitance lines.
In one embodiment of the invention, one of the supplemental capacitance lines is located outermost of a display region of the active matrix substrate for the pixel electrode which is located at a first peripheral side of the display region, the supplemental capacitance line being wider than the other supplemental capacitance lines and serving as a light-shielding line.
In another embodiment of the invention, a counter electrode signal is directly applied to the supplemental capacitance line which is located outermost of the display region and serves as the light-shielding line at a plurality of positions.
In still another embodiment of the invention, the liquid crystal display device further includes a light-shielding line which is formed for the pixel electrode located at a second peripheral side of the display region opposing the first peripheral side, in parallel with the supplemental capacitance line located outermost of the display region.
In still another embodiment of the invention, a counter electrode signal is input to the light-shielding line.
In still another embodiment of the invention, each of the switching elements is formed on a side of the corresponding scanning line nearer to a signal source from which a signal is input to the signal lines.
In still another embodiment of the invention, an insulting film is formed between the scanning lines and the pixel electrodes, and the thickness of the insulting film is about 2 xcexcm or more.
In still another embodiment of the invention, each of the pixel electrodes has a cutout formed above the corresponding scanning line.
In still another embodiment of the invention, each of the pixel electrodes includes two parts defined by the cutout formed above the corresponding scanning line and a plurality of connection portions connecting the two parts.
In still another embodiment of the invention, the plurality of connection portions are formed at opposite ends of the pixel electrode.
In still another embodiment of the invention, a width of at least one of the plurality of connection portions is different from a width of the other connection portions.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display device where production cost can be reduced without lowering the display quality.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.